In the transmission of digital signals and the reproduction of recorded digital signals it is necessary to regenerate the signals because of deviations from an original square shape due to various influences such as, e.g., band limitation and amplitude and phase deviations in the transmission characteristic. The regeneration is usually effected by bit-by-bit sampling of pulses at times at which no pulse edges are expected. This calls for the generation of a timing signal whose frequency and phase satisfy these requirements and which duly follows variations in pulse transit time.
For the generation of such a timing signal it is known, e.g., to obtain timing pulses from the edges of signals pulses and to use such timing pulses to control a variable oscillator. (See Marconi Review, No. 193, Second Quarter 1974, p. 93 et seq.) A necessary phase comparison can be carried out by means of one of two different circuits. One such circuit provides for the multiplication of sinusoidal or square voltage signals to be compared. However, in many codes suitable for digital transmission or recording, pulse edges occur only at irregular intervals. This is a drawback when such phase comparison circuitry is used because, in the absence of pulses, the control voltage does not remain at the last-available value. While phase comparison circuitry based on sampling and latching principles is not subject to this drawback, such latter circuitry requires pulses having very short duration; accordingly, these circuits are difficult to implement for high-frequency applications.